4.4 Composite structure of disturbance associated with grid-scale precipitation

In contrast to the experiment with Kuo scheme (subsection 3.4), preference of the propagation direction of the grid-scale precipitation activity in the experiment with moist convective adjustment scheme is not evident in either of the upper- or lower-level cooling experiment. In the following, in contrast to subsection 3.4, we present the composite structures for each of the eastward and westward propagating grid-scale disturbances in the upper-level experiment only. As for the composite structure of the lower-level experiment, athough magnitudes of anomalies are different, the overall characteristics do not differ greatly.

Structure of eastward propagating precipitation activity

Fig.4.10 is the composite structure referring to the eastward propagating grid-scale precpiptation of the upper level cooling experiment (adj-c). It is similar to that for the eastward moving precipitation activity of the upper level cooling experiment with Kuo scheme Fig.3.10. Namely, it has the characteristics of equatorial Kelvin waves; there exist the westward phase tilt exemplified as temperature anomaly in the longitude height section at the equator (upper right panel of Fig.4.10), the dominance of the east-west wind anomaly against the north-south (lower panels of Fig.4.10), and the equatorially confined pressure anomaly (lower panels of Fig.4.10). The difference from that with Kuo scheme is that the negative temperature anomaly appearing from the lower levels to the upper western side is intense and extends to the higher altitudes. The intense lower level cooling is considered to be caused by the effect of moist convective adjustment scheme.

The characteristics of the composite structure in the lower level cooling experiment (not shown here) are fairly similar to those for the upper level cooling experiment except that the intensities of anomalies are much weaker.

Fig.4.10: The same as Fig.3.10 but for composite structure referring to the eastward propagating grid-scale precipitation events for the upper-level cooling experiment with moist adjustment scheme (case adj-c). (upper left) longitude time section of precipitation at the equator. Unit is [kg m-2 s-1] Black dots indicate the points of reference chosen for the composite. (upper right) Longitude height cross section of temperature anomaly at the equator from the longitudinal mean (unit is [K]). Arrows indicate wind velocity anomaly from the longitudinal mean (the arrows at the lower right corner indicates reference magnitudes of [30m/s, 4x10^-6 s-1], respectively). (lower left) Surface pressure anomaly from the longitudinal mean (unit is [Pa]). Arrows indicate wind velocity anomaly from the longitudinal mean (the arrows at the lower right corner indicates reference magnitudes of [12m/s, 12m/s], respectively). (lower right) Height anomaly from the longitudinal mean in the upper troposphere (σ=0.23) (unit is [m]). Arrows indicate wind velocity anomaly from the longitudinal mean (the arrows at the lower right corner indicates reference magnitudes of [12m/s, 12m/s], respectively).

Structure of westward propagating precipitation activity

Fig.4.11 is the composite structure referring to the westward propagating grid-scale precipitation of the upper level cooling experiment (adj-c). It fairly resemble that for the westward moving precipitation activity of the lower level cooling experiment with Kuo scheme (Fig.3.11). Namely, the temperature anomaly is concentrated at the area of upward motion (precipitation area) and has no phase tilt as exemplified in the longitude height section at the equator (upper right panel of Fig.4.11), and the wind anomaly in the upper troposphere is isotropic (lower right panel of Fig.4.11). The features that is absent in the results with Kuo scheme are the strong negative temperature anomaly in the lower levels and the cyclonic rotation (anti-clockwise in northern hemisphere and clockwise in souther hemisphere) of the low level wind response around the equator. Similar to the low level negative temperature anomaly for the eastward moving precipitation activity mentioned above, the intense lower level cooling is considered to be caused by the effect of moist convective adjustment scheme. Emergence of cyclonic wind anomalies may result from the wider north-south extent of the precipitation activity (not shown here) in the experiment with moist convective adjustment scheme compared to those with Kuo scheme.

Similar to the results for eastward propagating disturbances above, the intensities of anomalies in the lower-level cooling experiment are weaker than those in the upper-level cooling experiment.

Fig.4.11: The same as Fig.3.10 but for composite structure referring to the westward propagating grid-scale precipitation events for the upper-level cooling experiment with mosit adjustment scheme (case adj-c).